1. Field of the Invention
The present invention relates to an active resin composition suitably employed in flip-chip mounting, to a surface mounting method, and to a printed wiring board produced through the surface mounting method.
2. Background Art
Conventionally, mounting of surface mount devices such as BAG parts on the surface of a printed wiring substrate has been carried out through a procedure including application of a flux onto the surface; mounting BGA parts on a printed wiring substrate; reflow soldering; washing off the flux; filling the space between the printed wiring substrate and the BAG parts with an under-filling resin; and curing the under-filling resin. As disclosed in Japanese Patent Application Laid-Open (kokai) No. 2004-152936 (claim 2), a flux known in the art contains, as an activating agent, a compound having a carboxylic acid group (e.g., rosin).
Meanwhile, a BGA part includes a plurality of mounted chips for enhancing device performance. Thus, the dimensions of such a BGA part have gradually over the years.
When the size of a BGA device increases, washing off the flux is hindered by the presence of BGA parts, and unremoved flux (i.e., flux residue) may remain. As a result, an activating agent contained in the flux residue causes problematic corrosion reaction during a subsequent step; i.e., a thermal curing of under-filling resin.
Meanwhile, as disclosed in Japanese Patent Application Laid-Open (kokai) No. 2002-237676, another type of the flux is a wash-less flux, which contains a less-corrosive activating agent having low activity and which requires no washing step. When such a wash-less flux is used, the wash-less flux generates decomposition gas during thermal curing of the under-filling resin, resulting in destruction of BGA parts, which is also problematic.
Furthermore, when the size of a BGA part increases, connection portions between BGA parts impedes filling the space with under-filling resin. Particularly when the surface of a printed wiring substrate has dents and protrusions (circuits, solder resist, etc.), the dents cannot be completely filled with the under-filling resin, in some cases, providing voids and unfilled portions. As a result, quality and reliability of products are considerably impaired. If such voids or the like are not found, when a subsequent step of thermally curing the under-filling resin is performed, the products cannot be repaired and must be discarded, thereby lowering product yield.